In this iris-photo onelist, it probably IS common
practice to give photographers credit for their hard work.

In addition, it IS common courtesy to do
so.

However, in the real world it is (unfortunately)
not always done, nor does it have to be; for many people do not care,
and there are no laws to stop it.

I am sure the person posting your picture only
wanted to give something to the list for people to enjoy, which is probably the
same reason you posted it originally - for others to enjoy. You should
take it complimentarily. I think it would have been quite different if he posted
it and tried to take credit for your work. THAT would be
offensive.